A nice thick mudset tiled floor can overcome some problems the new materials can't handle - the mudset floor is thick enough to act like a floating monolithic mass. Keep in mind that if you install a 'normal' subfloor and want to use something like cbu, you almost certainly will need to sister, and even then, it may not be enough. When you sister, keep in mind that you can let the sister ride higher than the existing joists, if it will help produce a nice flat, level floor. Letting the old stuff drop the 8 feet or so to the first floor risks some major damage, so I'd probably not consider that. The best place for quick answers on this type of question is: www.johnbridge.com , they live and die on tiling techniques. There are some here, but mostly plumbers. You'll get more and faster responses over there on tiling issue.